The most commonly used fluid level sensor is the variable resistor sensor utilizing a float to produce a resistance change in the variable resistor. As the float moves vertically with the fluid level, the electrical resistance of the sensor changes typically from 10 to 400 ohms. In most sensors, a sliding or moving contact attached to the float establishes a resistive circuit based upon the position of the contact with respect to a wirewound resistor or a thick film resistor printed on an insulating base or substrate.
Other approaches to fluid level detection include the use of resistors with large temperature coefficients, known as thermistors, located at various vertical positions in the fluid reservoir. As electrical power is applied to the resistors, the devices immersed in the fluid remain cool while those that are exposed to air will increase in temperature and produce a change in overall resistance of the device. Extensive signal conditioning and temperature compensation circuitry is typically required with such a sensor to create a usable signal. Fluid compatibility and manufacturing costs limit widespread acceptance of this type of device.
A vertical sensor with a sliding contact has been used in some automotive applications. Typically a float provides a contact point with respect to a resistor. The resistor is usually a wire helix wound about an insulating mandrel.
Examples of prior art fluid level sensors are shown in the following patents: Weaver, U.S. Pat. No. 4,920,798, Riley et al., U.S. Pat. No. 4,827,769, Guerrini et al., U.S. Pat. No. 4,702,107, Hoppert et al., U.S. Pat. No. 4,567,762, Coulange, U.S. Pat. No. 4,454,761, Bjork, U.S. Pat. No. 4,052,901, DeGiers, U.S. Pat. No. 2,484,690, and German Patent 2758379.
An example of thick film resistor technology used in a liquid level sensor is shown in Weaver, U.S. Pat. No. 4,920,798. The Weaver device includes a thick film resistive coated plate with a slidable contact member providing a resistance in proportion to the position of a float mounted on an adjacent rod.
Riley et al., U.S. Pat. No. 4,827,769, discloses a fuel level sensor including a soft steel substrate encased in porcelain with a cerment thick film resistive track deposited thereon. The Riley device provides a resistance proportional to float position.
Guerrini et al., U.S. Pat. No. 4,702,107, discloses a device for detecting the level of a liquid contained in a tank and includes a vertical bar and a float positioned according to liquid level. The Guerrini device establishes an inverse correlation between circuit resistance and fluid level.
Hoppert et al., U.S. Pat. No. 4,567,762 discloses a thermoelectric level detector including a meandering resistive path which repeatedly transverses the axis of the impedance element. The resistance of the meandering path is dependent upon temperature and provides a measure of liquid level.
Coulange, U.S. Pat. No. 4,454,761 discloses a liquid level detector including a slidable float and a winding disposed about the periphery of a rod. The float's vertical position, as defined by the liquid level, controls the resistance of the winding.
Bjork, U.S. Pat. No. 4,052,901 discloses a level detecting apparatus including an elongated flexible substrate transducer which is shorted out in the portion thereof subject to a threshold pressure. The resistance produced is inversely proportional to the liquid level.
DeGiers, U.S. Pat. No. 2,484,690 discloses an electric liquid level indicating device including a resistor element, a common conductor, a plurality of flexible sliding members between the resistor element and the common conductor, and a magnet attached to a float. The magnet shorts out the resistor at the liquid level by attracting a corresonding flexible sliding member.
German Patent 2758379 discloses a potentiometer for indicating liquid levels including a chain of resistors connected to reed relays. The relays are actuated by a permanent magnet attached to a float. The resistance of the potentionmeter is proportional to the liquid level.
An improved liquid level sensor with highly reliable components yet economical to manufacture is needed.